The invention relates to improvements in methods of and in apparatus for applying coating material to substantially plate-like substrates, especially to circuit boards. More particularly, the invention relates to improvements in methods of and in apparatus of the type described in U.S. patent application Ser. No. 095,319, filed Sept. 10, 1987, and now U.S. Pat. No. 4,836,137 granted Jun. 6, 1989 to Heine et al.
The apparatus of Heine et al. comprises a continuously driven transporting unit which advances successive substrates of a series of substrates along an elongated horizontal path beneath an applicator which sprays coating material (such as photosensitive resist or solder resist) onto the upper sides of successive substrates. The applicator is an electrostatic applicator with a rotary atomizer for coating material. Freshly coated substrates are advanced into a drying unit which expels moisture from the applied coating material. The patented apparatus is equipped or combined with an automatic feeder which supplies substrates to the transporting unit ahead of the spray coating station. Such apparatus can process large numbers of substrates per unit of time. However, only one side of each substrate is provided with coating material so that, if both sides of each substrate are to be provided with coats of a flowable material, it is necessary to deliver the substrates to the patented apparatus for a second time but in inverted condition so that the uncoated sides of the redelivered substrates face upwardly during transport past the coating station. A drawback of such mode of coating both sides of successive substrates is that one of the coats is dried twice. Moreover, the interval of time which is required to coat both sides of a given number of substrates is rather long because the substrates can be inverted only after the initially applied coats are sufficiently dry and stable in order to avoid damage as a result of contact with the transporting unit during renewed advancement beneath the coating means.
Published German patent application No. 37 35 798 of Grah et al. discloses an apparatus which can simultaneously apply coats to both sides of each of a series of successive substrates. The inverters propose to advance the substrates in suspended condition so that each of their sides is located in a vertical plane. The application of coating material to one side of each of a series of successive substrates precedes the application of coating material to the other side, and the applied coats are thereupon dried simultaneously downstream of the second coating station. If both coating stations are to be disposed at the same side of the path for suspended substrates, it is necessary to equip the apparatus of Grah et al. with a turn-around device which changes the orientation of successive substrates through 180.degree. by turning each substrate about a vertical axis. The substrates are delivered into the range of the transporting means by h and, and it is necessary to manually detach successive treated substrates from the transporting unit. Successive substrates of the series of suspended substrates are separated from each other by vertically extending gaps which must be relatively wide if the substrates are to be turned about vertical axes. The gaps permit large quantities of sprayed coating material to bypass the advancing substrates. Moreover, the coating action is not predictable because droplets or streamlets of coating material tend to flow along the respective sides of the substrates under the action of gravity. Relatively large droplets of photosensitive resist exhibit a pronounced tendency to flow along the respective sides of the advancing substrates. The same applies for solder resist, especially if the viscosity of such coating material is relatively low. An additional drawback of the apparatus of Grah et al. is that the transporting unit undergoes pronounced contamination, especially if the applicators include devices for electrostatically spray coating the sides of suspended substrates.
Another spray coating apparatus for plate-like substrates is disclosed in U.S. Pat. No. 4,867,099 to Heine et al.